1. Field of the Invention
The present invention relates to a method for detecting a deviation of a compression pressure of one cylinder from that of another cylinder of an internal combustion engine, and to a processing unit for carrying out the method.
2. Description of the Related Art
Fuel injection systems facilitate the metering of the fuel required for a combustion in an internal combustion engine, with the aid of one or multiple injectors (also referred to as fuel injectors or injector nozzles). In the case of direct gasoline injection or common-rail injection, the fuel is injected directly into the combustion chamber. The metered fuel quantity is of crucial importance in terms of quality of combustion and, therefore, the consumption and exhaust behavior of the internal combustion engine.
However, the metered fuel quantity is affected by properties of the injector itself. Due to manufacturing tolerances and wear, which may occur in the injectors employed within an internal combustion engine, the fuel quantity (i.e., quantity of fuel injected) metered by the injectors may vary.
With the aid of a run-up test, it is possible to easily check without disassembly the quantity deviation of the installed injectors of an internal combustion engine, even in a motor vehicle in the end customer's possession. The particular conditions in the workshop environment allow for modified operating modes as well as permanently defined, stable operating points. This is frequently not possible during driving operation, since the control of the internal combustion engine must implement the driver's input. Different injection quantities may be detected during the run-up test of the internal combustion engine by accelerating the internal combustion engine for a fixed period of time (so-called run-up), starting from a specific starting rotational speed by setting a defined setpoint injection quantity for all injectors but one. In this test, accuracy functions, balance functions and correction functions are suppressed, if necessary, using values learned from the past, in order to detect the actual state as clearly as possible. The target rotational speed achieved is then measured. The injector not actuated is then continuously varied. If the target rotational speeds differ, or if a target rotational speed deviates significantly from the average of the other target rotational speeds, then a deviation of the actual injection quantity from the setpoint injection quantity may be inferred, if the remaining parameters, which affect the torque, in this case in particular the compression pressure (pressure inside a cylinder which is generated by the piston stroke), are the same for all cylinders.
It is desirable, therefore, to be able to check in a simple manner whether the compression pressure, as a definitive parameter affecting the torque, is the same for all cylinders.